There has been an increasing demand for porous materials in industrial applications such as gas storage, separations, catalysis and conductive materials. Some of the advantages of organic porous materials over their inorganic or metal-organic counterparts, include: lighter molecular weight, easier to functionalize, and generally have better kinetic stability. Moreover, organic porous materials are more environmentally friendly than comparable frameworks.
Current methods to introduce porosity into polymeric structures are largely based on processing the polymers under certain conditions, or by preparing the polymers from colloidal systems. All glassy polymers contain some void space (free volume), although this is usually less than 5% of the total volume. It is possible to “freeze-in” up to 20% additional free volume for some glassy polymers with rigid structures by rapid cooling from the molten state below the glass transition temperature, or by rapid solvent removal from a swollen glassy polymer. High free volume polymers are currently used in industrial membranes for transporting either gases or liquids. The voids in these materials, however, are not interconnected and therefore reflect a low accessible surface area as determined by gas adsorption. Moreover, the pore structure is irregular and not homogeneous.
Another existing class of porous organic materials includes polyacetylenes containing bulky substituent groups. The high gas permeabilities of poly(l-trimethylsilyl-1-propyne) (“PTMSP”) has been observed since 1983. This material contained a large free volume (˜30%), and was able to separate organic compounds from gases or water. The stability of PTMSP is limited by its rapid loss of microporosity due to non-uniform pore structure, exposure to heat, oxygen, radiation, or UV light, or any combination of the above.
Recently, polymers of intrinsic microporosity (PIMs) were shown to have exceptional porosity for organic polymers. As measured by gas adsorption, PIMs were reported to contain relatively high surface areas (430-850 m2/g). The porosity of PIMs is likely due to their highly rigid and contorted molecular structures which inhibit efficiently packing in space. PIMs, however, display marked hysteresis at low pressures.